feat: easier steady-state vs transient period selection

[wpbonelli]

We should think about some way to simplify the selection of steady state and transient stress periods. This is relevant to e.g. the storage package

Originally posted by @mjr-deltares in #2352 (comment)

Hey @jlarsen-usgs , I looked at the failing test and realized I was having similar issues. The storage package is created with:

        ... 
        steady_state={0: True, 1: False},
        transient={0: False, 1: True},
    )

which the splitter can't process into something valid. Reducing it to:

    steady_state={0: True},
    transient={1 : True},
)

works as intended.